A known frictional propulsion device for an omni-directional vehicle comprises a main wheel including an annular core member and a plurality of driven rollers (free rollers) fitted on the annular core such that each driven roller may be freely rotatable around the tangential direction of the annular core member at the corresponding position, and a pair of drive disks provided on either side of the main wheel each including a disk member and a plurality of drive rollers (free rollers) arranged along the outer periphery of the disk member and engaging the driven rollers of the main wheel so as to be each rotatable around a rotational center line at an angle to both the lateral direction and the radial direction. See JP2011-63209A and JP2011-63252A, for instance.
In the disclosed frictional propulsion devices, the drive disks are rotatably supported by the body frame of the omni-directional vehicle so that the driven rollers are interposed between the drive rollers from either side, and the lower most driven rollers of the main wheel contact the road surface.
In such a frictional propulsion device, only the driven rollers located in the lower most part of the main wheel contribute to the propulsion of the omni-directional vehicle while the remaining driven rollers rotate idly around the core member without contributing to the propulsion of the vehicle.
In the structure disclosed in JP2011-63209A, the drive disks and the main wheel are arranged concentrically so that the drive side contact circle defined by the points on the drive disk at which the drive rollers contact the driven rollers is equal in diameter to the driven side contact circle defined by the points on the main wheel at which the driven rollers contact the drive rollers. Therefore, a large part of the driven rollers rotate without contributing to the propulsion of the vehicle so that a significant power loss is caused. Also, as these contact circles are identical to each other in diameter, about the same number of drive rollers as the driven wheels on the main wheel are required to be provided on each drive disk. The main wheel is required to have a relatively large diameter in view of achieving the desired terrain traversability of the main wheel, and this in turn causes an increase in the number of drive rollers. The need for a large number of drive rollers causes an increase in the manufacturing cost and an increase in the weight.
In the structure disclosed in JP2011-63252A, the central axial line of each drive disk is tilted with respect to the central axial line of the main wheel so that only those drive rollers located in a lower most part engage the driven rollers, and the number of drive rollers that are idly driven can be reduced. However, as the drive side contact circle and the driven side contact circle are still identical in diameter to each other, a relatively large number of drive rollers are required in proportion to the size of the main wheel, and this causes an increase in the manufacturing cost and an increase in the weight.